Relay stations for microwave communication systems



April 16, 1957 R. M. SPRAGUE RELAY STATIONS FOR MICROWAVE COMMUNICATIONSYSTEMS Filed NOV. 19, 1948 INVENTOR Ross/21' M. SPRAGUE A Ton/vs! atentI RELAY STATIONS FGR MICRGWAVE CQCATION SYSTEMS Robert M. Sprague,Waban, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass,:1 corporation of Delaware Application November 19, 1948, Serial No.60,941

3 Claims. (Cl. 250-45) This invention relates to microwave communicationiystems, and more particularly to a relay station there- One of theobjects of the present invention is the proprovision of a microwaverelay station in which signal distortion is held to a minimum.

Another object of this invention is the provision of a microwave relaystation the carrier of which is frequency stabilized within closelimits.

These, and other objects of the present invention, which will becomemore apparent as the detailed description thereof progresses, areattained, briefly, in the following manner.

A portion of the output of a local, highly stabilized, microwavegenerator is mixed with a frequency-modulated carrier received from aremotely located transmitting station to derive anintermediate-frequency wave including the remotely incorporatedintelligence. Another portion of the output of said local, highlystabilized, microwave generator is mixed with a portion of the output ofanother local microwave generator, which is to be stabilized, to deriveanother intermediatefrequency wave.

The two intermediate-frequency waves thus obtained are mixed to derive athird intermediate-frequency wave which also includes the aforesaidremotely incorporated intelligence.

The last-named intermediate-frequency wave is demodulated to obtain aunidirectional voltage, the magnitude and sense of which are functions,respectively, of the magnitude and sense of any deviation of thefrequencies of the carrier of said second-named local microwavegenerator and said received, frequency-modulated carrier from the meanfrequencies thereof. Said voltage contains two components, a directcurrent component corresponding to any drift of the local microwavegenerator, and an alternating current component corresponding to theintelligence remotely incorporated in the received carrier.

This voltage is applied, through a high-pass filter, to a suitablefrequency control which is connected to the local microwave generator tofrequency modulate the same and thereby impress thereon the intelligenceincorporated in the received carrier.

The second intermediate-frequency wave obtained as above is alsodemodulated to obtain another unidirectional voltage similar to thatjust described. This voltage is also applied to the aforementionedfrequency control, through a low-pass filter, to stabilize the meanfrequency of the local microwave generator.

In the accompanying specification there shall be described, and in theannexed drawing shown, an illustrative embodiment of the relay stationof the present invention. It is, however, to be clearly understood thatthe present invention is not to be limited to the details herein shownand described for purposes of illustration only, inasmuch as changestherein may be made without the exercise of invention, and within thetrue spirit and scope of the claims hereto appended.

In said drawing, the single figure is a schematic diagram of a microwaverelay station assembled in accordance with the present invention.

Referring now more in detail to the aforesaid illustrative embodiment ofthe present invention, with particular reference to the drawingillustrating the same, the numeral 10 designates an electromagnetic hornadapted to receive a remotely generated, frequency-modulated carrierwave w +w wherein m represents a microwave carrier of a centerfrequency, for example, of 4,000 megacycles per second, and rurepresents an instantaneous frequency deviation, for example, of 5megacycles per second, effected upon the carrier by the intelligenceintended to be transmitted.

The horn 10 is coupled, for example, to the so-called H arm 11 of a waveguide assembly 12, known as a magic T. Such a wave guide assemblycomprises, in addition to the H arm 11, a pair of side branches 13 and14- extending in opposite directions outwardly from said H arm,perpendicular thereto, and a so-called E arm 15 extending outwardly fromsaid side branches, mutually perpendicular to said side branches and tosaid H arm, said H and E arms and said side branches all extending froma common junction 16. As shown in the drawing, the H arm 11 recedes fromthe observer for a short distance from the junction 16, then bends tothe left, and is then twisted through an angle of The side branches 13and 14 are terminated in oppositely disposed crystals 17 and 18, betweenone side of each of which and the magic T itself capacitances 19 and 20exist, said magic T being grounded, and said crystals being connected,in series with a source 21 of direct current and a resistor 22, toprovide parallel outputs between a point 23 on their series connectionand ground. The source 21 of direct current is for the purpose ofoperating the crystals at a favorable point along their characteristiccurve, and the series circuit is for the purpose of passing equalcurrents through said crystals to assure their similar action eventhough both possess dissimilar characteristics.

The E arm 15 of the assembly 12 is coupled through a wave guide section24 to an oscillator 25 adapted to generate microwave energy of afrequency w which may, for example, be of 4,100 megacycles per second.The oscillator 25 should be extremely stable and is, preferably, of thecavity-controlled klystron type.

For an understanding of the operation of the magic T 12, assume that thereceived modulated carrier w +w is traveling along the H arm 11 with itselectric vector pointing upwardly. As this energy reaches the twist insaid H arm, its vector is rotated through 90 and points toward theobserver, and, as this energy reaches the bend in said H arm, itselectric vector is again rotated through 90 and points toward the left,which is its position when it arrives at the junction 16. At saidjunction, the wave splits into two constituent waves, one travelingalong the side branch 13 toward the crystal 17 and the other travelingalong the side branch 14 toward the crystal 13, both constituent waveshaving their electric vectors pointing in the same direction, namely,toward the left.

Now further assume that the locally generated carrier Wave w istraveling along the E arm 15 from the oscillator 25 with its electricvector pointing upwardly. As this energy reaches the junction 16 of themagic T 12, it splits into two constituent waves, one having it electricvector pointing toward the right, traveling alongthe side branch 13toward the crystal 17, and the other having its electric vector pointingtoward the left, traveling along the side branch-14 toward the crystal18'.

to the E arm 34 of the magic T 31.

Thus, there arrives at the crystal 17 two constitutent waves whoseelectric vectors point in opposite directions and which therefore may beconsidered out of phase, and there arrives at the crystal 18 twoconstituent waves whose electric vectors point in the same direction andwhich therefore may be considered in phase. Therefore, there is producedat'the crystal 17 a beat-frequency wa-ve whose frequency corresponds tothe difference between frequencies of the constituent waves, and whosephase may be considered negative because its component waves are out ofphase, while, at the crystal 18, there is produced a beatfrequency wavehaving the same frequency as said firstnamed beat-frequency wave, andhaving a phase which may be considered positive because its componentwaves are in phase. However, as to each other, the two beatfrequencywaves thus obtained are in' phase, because the crystals are oppositelypoled, and set up equal voltages of like polarity across the crystals.Inasmuch as parallel outputs are taken from these crystals, additionoccurs, and an intermediate-frequency wave w w appears between the point23 and ground. In the case under consideration, the mean frequency w ofthis intermediate-frequency wave is 1-00 megacycles per second.

This intermediate-frequency wave is applied to :a broadband amplifier 26whose center frequency corresponds to the difference w between thenormal mean frequencies of the remotely and locally generated carrierwaves hereinbefore referred to.

The output of the amplifier 26, w w is applied to a mixer 27. A secondinput to the mixer 27 is derived as follows. 7

A portion of the output ca of the oscillator 25 is fed, through adirectional coupler 28, to a wave guide section 29 which, in turn,is'coupled to the H arm 30 of another magic T wave guide assembly 31.This assembly, like the assembly 12-, includes, in addition to the H arm30, side branches 32 and 33 and an E arm 34, all extending from a commonjunction 35. The side branches 32 and 33 are terminated inoppositely-disposed crystals 36 and 37 connected in series with a source38 of direct current and a resistor 39. Capacitances 4t and 41 existbetween one side of each crystal and the assembly, and the entireassembly is grounded.

An oscillator 42, adapted to generate a modulated microwave carrier w -wwherein w represents a center frequency, for example, of 4,260megacycles per second, and represents an instantaneous deviation whichis less than the deviation tu is coupled to an electromagnetic horn 43whereby said energy may be radiated into space. This oscillator is,preferably, of the magnetron type, and it is this oscillator that it isdesired, in accordance with the principles of the present invention, tofrequency modulate with the intelligence incorporated in the carrierreceived from the previous-mentioned, remotely-located transmitter. Itis this oscillator, also in accordance with the principles of thepresent invention, Whose mean frequency it is desired to stabilize.

For the above purposes, a portion of the output of the oscillator 42 isapplied, through a directional coupler 44, to a wave guide section 45which, in turn, is coupled it will be noted that there is applied to themagic T 31 a portion of the carrier w generated by the oscillator 25 anda portion of the carrier (d -w generated by the oscillator 42. Theoperation of the magic T 31 is similar to that above described inconnection with the magic T 12 and there is therefore produced, betweena point 46 in the series conneotion of the crystals as and 37 andground, a second intermediate-frequency wave tu -flu, of a centerfrequency a corresponding to the difference between the carrier wavesplified output is applied as a second input to the mixer 27.

In the mixer 27 there is produced a third, frequencymodulated,intermediate-frequency wave nt -02, wherein w =w w and w :-w -w in otherwords, the difference between the first frequency-modulated,intermediate-frequency wave w w and the second intermediate-frequencywave w w In the case under consideration, the difference centerfrequency w is 60 megacycles per second. 7

This third intermediate-frequency wave is amplified in a broad-bandamplifier 48, and the amplified output is applied to a limit-er 49 toremove undesired amplitude modulation.

The output of the limiter 49 is applied to a frequencysensitive circuit,for example, a discriminator 50, the output of Which is a unidirectionalvoltage whose magnitude and sense are functions, respectively, of themagnitude and sense of any eviation of the .thirdintcrmediatefrequencywave from its mean frequency w Such devirent component and the deviationco produces an alternating current component. a V

In any event, the output of the discriminator 5%) is applied to ahigh-pass filter 51 which, in turn, is coupled to any suitable tuningmechanism 52 for controlling the frequency of the oscillator 42. Thus,the direct current component of the output of the discriminator 50 isremoved, .and the alternating current component of said output operatesthe frequency control 52 to frequencymodulate the oscillator 42 andthereby impress thereon the intelligence originally received.

The foregoing will be better understood from a consideration of thefollowing. That portion of the system which includes the wave guide 45and the magic T assembly 31 constitutes the [3 or degenerative circuitof a feedback loop. That portion of the system which includes theamplifier 48, limiter 49, discriminator 5G, filter 51,

frequency control 52 and oscillator 42-constitutes the n oramplification circuit of the loop. The ,8 circuit contains no elementsthat will introduce substantial distortion and the gain of this circuitis unity. The deviation of the n circuit herein shown is w,, which issmall, and generates little distortion, and the gain of this circuitdepends on the sensitivity of the discriminator St) and the frequencycontrol 52, and determines the loop gain of the system as a whole. Thedeviation o of the oscillator 42 is where x is the loop gain, so thatif, for example, 'the'loop gain is 9, the deviation o of the oscillator42 is 9.9m and the difference w, between the deviations ca and o is 0.1It will thus be seen that the higher the loop gain, the less thereduction of deviation at the relay station, and, consequently, thegreater the reduction "of distortion.

Now, it is also desirable to stabilize the oscillator 42, and,,for thispurpose, resort may be had to the following expedient.

A portion of the output of the second intermediatefrcquency amplifier 47is also applied to a discriminator 53 to obtain a unidirectional outputwhose magnitude and sense are functions, respectively, of the magnitudeand sense of any deviation of the frequency of theintermediate-frequency wave w from its normal frequency corresponding tothe difference between the frequencies of the carriers ar and wObviously, a drift in the frequency of the carrier w will produce adrift in the frequency of the wave m The voltage obtained as a result isapplied to a low-pass filter 54 to remove any alternating currentcomponent and the direct current output of said filter is applied to thetuning or frequency control 52 of the oscillator 42.

This completes the description of the aforesaid illustrative embodimentof the present invention. It will be noted from all of the foregoingthat the present invention provides a microwave relay station in whichdistortion is held to a minimum and in which the can'ier generated atthe relay station is frequency stabilized within close limits under thecontrol of the received carrier or under the control of a local feedbackcircuit.

Other objects and advantages of the present invention will readily occurto those skilled in the art to which the same relates.

What is claimed is:

1. A relay station for a microwave communication system comprising:means for locally generating a first carrier wave; means for locallygenerating a second carrier wave; means for receiving a third carrierwave; all said carrier waves being of different frequencies; means,receptive of said second and third carrier waves, for deriving therefroma first intermediate-frequency wave; means, receptive of said first andsecond carrier Waves, for deriving therefrom a secondintermediatefrequency wave; means, receptive of said first and secondintermediate-frequency waves, for deriving therefrom a thirdintermediate-frequency wave; means, receptive of said thirdintermediate-frequency wave, for deriving therefrom a unidirectionalvoltage the magnitude and sense of which are functions, respectively, ofthe magnitude and sense of the difference between any instantaneousdeviations of the frequencies of said first and third carrier waves fromtheir initial frequencies; and means, receptive of said unidirectionalvoltage and connected to said first-named means, for altering thefrequency of said first carrier wave to modulate the same in response tosuch frequency I wave; means, receptive of said first and secondintermediate-frequency waves, for deriving therefrom a third, frequencymodulated intermediate frequency wave; means, receptive of said thirdintermediate-frequency wave, for deriving therefrom a unidirectionalvolage the magnitude and sense of which are functions, respectively, ofthe magnitude and sense of any deviation of the frequency of said thirdcarrier Wave from its mean frequency; means, receptive of saidunidirectional voltage and connected to said first-named means, formodulating the frequency of said first carrier wave in response to suchfrequency deviation.

3. A relay station for a microwave communication system comprising:means for locally generating 'a first carrier wave; means for locallygenerating a second carrier wave; means for receiving a third carrierWave; all of said carrier Waves being of different frequencies; means,receptive of said second and third carrier Waves, for deriving therefroma first intermediate-frequency wave; means, receptive of said first andsecond carrier waves, for deriving therefrom a secondintermediate-frequency wave; means, receptive of said first and secondintermediatefrequency waves, for deriving therefrom a thirdintermediate-frequency Wave; means, receptive of said thirdintermediate-frequency wave, for deriving therefrom a unidirectionalvoltage the magnitude and sense of which are functions, respectively, ofthe magnitude and sense of any deviation of the frequency of said firstcarrier wave from its initial frequency; means, receptive of said secondintermediate-frequency wave, for deriving therefrom a unidirectionalvoltage similar to said first-named unidirectional voltage; and means,receptive of both said unidirectional voltages and connected to saidfirst-named means, for altering the frequency of said first carrier waveto compensate for any such frequency deviation and to modulate saidfirst carrier wave in accordance with any intelligence incorporated insaid third carrier wave.

UNITED STATES PATENTS References Cited in the file of this patent1,731,264 Potter Oct. 15, 1929 2,334,189 Goldstine Nov. 16, 19432,341,649 Peterson Feb. 15, 1944 2,460,789 Thompson Feb. 1, 19492,462,841 Bruck et a1. Mar. 1, 1949

